1. Field of the Invention
This invention relates to microwave feed networks and more particularly to a microwave distribution network for dividing and combining a number of microwave signals for coupling to a plurality of antenna elements of a phased array antenna.
2. Description of the Prior Art
Phased array antennas typically have a plurality of radiating elements along a path. Each radiating element is fed with a microwave signal having a particular amplitude and phase. In the general case, a phase shifter is provided between a microwave signal and each element so that the phase of the microwave signal at each element may be controlled. In order to reduce the number of phase shifters required to drive a phased array antenna in limited scan applications, subarrays are fed with a microwave signal through a single phase shifter. The subarray which may comprise several antenna elements, such as two or greater, is fed with an antenna feed network where the microwave signal, after leaving the phase shifter, is divided and the signal power is prorated in a predetermined manner among the subarray antenna elements. The amount of power distributed to each antenna element is also known as the illumination function and by providing a predetermined illumination function such as a sin x/x pattern, a beam of a predetermined shape may be generated in the far field. The power distributed to the radiating elements of the subarray may also be adjusted to provide a Taylor, uniform, Chebycheff, or binomial function which is well known in the art. The subarrays of a phased array antenna may be spaced apart by a predetermined distance or may be overlapped with other subarrays. With overlapped subarrays, common antenna elements are used for each subarray and the antenna feed network must combine the microwave signals for each subarray together before feeding the common antenna element.
By overlapping subarrays and tailoring the subarray pattern to closely match the selected scan coverage region of the antenna, grating lobes and side lobes outside the selected scan coverage region may be suppressed.
In U.S. Pat. No. 4,321,605 which issued on Mar. 23, 1982 to Alfred R. Lopez, an array antenna is described. In FIG. 4, a plurality of 2N first transmission lines are shown for supplying wave energy to one of the element groups. Second transmission lines having a signal input end intersect a selected number of first transmission lines before being terminated at its other end. Directional couplers are provided for coupling the second transmission lines to the first transmission lines.
In U.S. Pat. No. 4,143,379 which issued on Mar. 6, 1979 to H. A. Wheeler, an antenna feed network is shown such as in FIGS. 3 and 7 for feeding a phased array antenna having overlapped subarrays. In FIG. 3, an 8 element subarray is shown being fed at input port 31d from one phase shifter wherein elements 2 and 7 in the subarray are not fed to provide a resulting sin x/x illumination pattern. The adjacent subarray, being fed at input port 31c, overlaps the subarray fed by input port 31d by 6 elements.
FIG. 7 shows a modular coupling network 94d with input port 31d which, when combined with a number of similar modules, provides a coupling network to several overlapped subarrays. In FIG. 7, branch line directional couplers, shown in more detail in FIG. 5, are used to divide the power further from power divider 36d. Zero db couplers are shown such as 82.sub.a through 82.sub.e for providing crossover networks in a single wiring plane. A more detailed description of the zero db couplers is found in column 5 and FIG. 6. As shown in FIGS. 3 and 7, the microwave signal from input port 31d is divided by power divider 36d and fed over two transmission lines to antenna element terminals 110d and 112d. Signals for other elements of the subarray are coupled from the two transmission lines feeding elements 110d and 112d.
In U.S. Pat. No. 4,041,501 which issued on Aug. 9, 1977 to R. F. Frazita et al., a phased array antenna system is described using coupling circuits to reduce the number of phase shifters required. In FIG. 6 phase shifter 13a provides a microwave signal to power divider 48 which divides the signal and provides it on transmission lines 50 and 52 to antenna elements 12a through 12d. In addition, couplers 58 and 60 couple microwave energy from transmission lines 50 and 52, respectively, onto transmission lines 56 and 54, respectively. Transmission lines 56 and 54 have attenuators 66 and 64 in the line to couple a predetermined amount of microwave energy to other antenna elements by way of couplers 58 and 60, respectively. As may be seen in FIG. 6, each phase shifter 13a through 13f provides a microwave signal to a respective module which in turn directly drives its antenna elements and at the same time couples power off to other antenna elements in other modules so as to provide overlapping subarrays with each subarray having a predetermined illumination function. Frazita et al. also shows in FIG. 2 and discusses in column 4, at lines 17-36, the spacing of the subarrays so that the grating lobe does not enter the subarray pattern when the array is scanned.
In U.S. Pat. No. 3,803,625 which issued on April 9, 1974 to J. T. Nemit, a network approach is described for reducing the number of phase shifters in a limited scan phased array. FIG. 5 of Nemit shows a three element subarray being fed by a microwave signal from phase shifter 29. The subarray and an adjacent subarray are overlapped by one antenna element. For example, element 20 is fed with microwave signals from phase shifters 28 and 29 and combined together by coupler 25.
While all of the prior art networks employ an overlapping subarray approach to reduce the number of phase shifters and provide suppression of grating lobes and side lobes outside the scan coverage region, each has certain characteristics which limits its usefulness or practicability. For example, in the network described by Lopez in U.S. Pat. No. 4,321,605, the antenna element on one end of the subarray is fed from the network input through a singular path of four couplers, while the antenna element on the opposite end of the subarray is fed through a singular path of eight couplers, and an antenna element in the middle of the subarray is fed through seven different paths and twelve couplers. The extreme asymmetry and multiple "sneak" paths make the design of this network quite complex and the physical realization of the desired element amplitudes and phases difficult.
In U.S. Pat. No. 4,321,605, Lopez points out that the usefulness of the Frazita network in U.S. Pat. No. 4,041,501 is limited by its frequency sensitivity, while the practicability of the Wheeler network in U.S. Pat. No. 4,143,379 is limited by the circuit complexity, resulting from the high number of network interconnections and crossovers.
Nemit in U.S. Pat. No. 3,803,625 describes only a 3 element subarray network in his patent. He suggests that a more ideal subarray pattern could be achieved by feeding a larger number of elements; however, as Frazita points out in U.S. Pat. No. 4,041,501, Nemit does not describe in U.S. Pat. No. 3,803,625 a practical technique for doing this.
Another important parameter that must be considered in determining the usefulness or practicability of a particular network is the network loss. All the prior art networks have an inherent loss over and above the normal ohmic conductor loss due to power absorbed in circuit attenuators and/or terminating loads which are dependent on the subarray illumination function and the particular set of hybrid coupling values selected. No reference is made in any of the prior disclosures to this loss or how the network may be designed to minimize it.
It is therefore desirable to provide a number of antenna feed networks for coupling microwave signals to overlapped subarrays of antenna elements in a phased array antenna to reduce the number of phase shifters required for limited scan application, while suppressing side lobes and grating lobes in the out-of-scan coverage region.
It is further desirable to provide a number of antenna feed networks for feeding overlapped subarrays of antenna elements with a subarray of 4 or more antenna elements with any desired subarray illumination function.
It is further desirable to provide a number of antenna feed networks for feeding overlapped subarrays of antenna elements with various degrees of phase shifter reduction, for example, 50 percent, 66 percent, 75 percent or more.
It is further desirable to provide antenna feed networks that have no loss over the normal ohmic conductor loss for the case where the subarray illumination function element weights are all in phase and a minimum loss for the case where the subarray illumination function element weights have any arbitrary phase.
It is further desirable that the networks have substantially equal path lengths between an input and any antenna element in the corresponding subarray to provide broadband performance.
It is further desirable that these networks have unique, singular propagation paths from an input to any element in the corresponding subarray to simplify network design and adjustment of the element amplitude and phase weight.
It is further desirable to provide a network having a distributed corporate arrangement of power dividers and combiners to minimize the number of crossovers and circuit complexity.
It is further desirable that the networks be modular in design and have two dimensional planar network topology for application to low-cost, practical circuit strip-line and microstrip construction technology.
It is further desirable to provide an antenna feed network which utilizes Wilkenson dividers.
It is further desirable to provide an antenna feed network module which when coupled with other modules and to antenna elements will provide equal microwave signal length to each antenna element and will drive a plurality of overlapped subarrays.